1. Field of the Invention
The present invention relates to structures of field-effect transistor (FET).
2. Related Background Art
As FETs realizing ultrafast operations, there have been known those employing a pulse-doped structure in a channel layer. The pulse-doped structure is a structure in which a channel layer with a high doping concentration is formed directly under an undoped layer (cap layer). The cap layer has a predetermined thickness from the substrate surface on which a gate electrode is formed on the cap layer. FETs having the pulse-doped structure are disclosed, for example, in U.S. Pat. No. 4,163,984 or Japanese Patent Application Laid-Open No. HEI 6-310535.
In n-type FETs having the pulse-doped structure, a long gate effect may occur. The long gate effect is a phenomenon in which, the effective gate length enhances due to a depletion layer on the drain electrode side (where the gate bias is a small negative value). For suppressing the long gate effect, the FET disclosed in Japanese Patent Application Laid-Open No. HEI 4-225533, for example, employs a double-pulse-doped layer structure. The double-pulse-doped layer structure is a structure further comprising a pulse-doped layer (additional doped layer) which is disposed over a channel layer. The channel layer and the additional doped layer are separated from each other by a predetermined distance by an undoped layer (a part of a cap layer). The additional doped layer inhibits the depletion layer from spreading toward the channel layer.